There are numerous applications where it is necessary or desirable to disconnect one or more loads from one power supply and to connect the load(s) to another power supply with minimum disruption in the load current. In aircraft and aerospace applications, a load transfer may be required between power sources external to and on-board the aircraft or between separate on-board power sources. Prior on-board power sources have been typically of the constant speed type having a hydromechanical constant speed drive coupled to the engine of the aircraft which converts the variable-speed motive power produced by the engine into constant-frequency AC power for the loads. Such a system is sometimes referred to as an integrated drive generator (or IDG).
An alternative to the foregoing power source that does not use a constant speed drive is referred to as a DC link power generating system wherein DC power on a DC link is converted into constant-frequency AC power. One type of DC link power generating system is known as a variable-speed, constant-frequency (VSCF) generating system which includes a synchronous generator coupled directly to the aircraft engine and a power converter which converts the variable frequency output of the generator into constant frequency power for the loads. In multiple aircraft having engines each engine typically drives a separate VSCF system (sometimes referred to as a "channel") and the system outputs are coupled to a load bus through contactors. Interest in VSCF systems has increased of late owing to the push to design "all electric" aircraft in which the use of mechanical, hydromechanical and hydraulic components is minimized.
In a multiple-channel VSCF system of the above type, it may be necessary to change the source of power to the load bus from one channel to another or between an external AC source, such as a ground power cart, and one of the channels.
Recker, et al, U.S. Pat. No. 4,937,462 discloses a no-break power control for a VSCF power generating system. The system senses the deviation of a parameter of the power developed by an inverter of the VSCF system relative to a parameter of the power developed by an AC power source and controls the inverter in accordance With such deviation to cause the parameter of the power developed thereby to approach the parameter of the power developed by the AC power source. The inverter in the VSCF system and the AC power source are connected in parallel across one or more loads when the parameter deviation is within a certain range. Either the inverter or the AC power source is thereafter disconnected at a certain time after the two have connected in parallel to complete the power transfer.
The Recker, et al. control is effective to bring the inverter and the AC power source into synchronism so that parallel connection across a load is possible. However, during the time that the inverter and the AC power source are connected in parallel, (which may be, for example, up to 200 ms. for 400 a hz. power system) the inverter or the AC power source may drift in frequency and output voltage, resulting in increasing circulating current during the parallel time. Also, some initial voltage magnitude and phase errors may be experienced.
The possible voltage magnitude, frequency and phase errors between a pair of AC sources can result in undesirable circulating currents and power transfers. For example, voltage mismatches in the ten volt range can result in 53 amps of circulating reactive current while a phase mismatch of up to 30.degree. can result in up to 440 amps of real current being passed between the two sources. When a solid state source such as a VSCF generator system lags a further AC source in phase, the former receives real power which must be dissipated or stored. Dissipation requires an active suppressor which adds weight and complexity to the VSCF system. Stored energy may be handled to some extent by the DC link filter capacitor typically used in DC link systems. However, this capacitor is usually only sized to handle ripple current and must therefore be made much larger to store the required worst case real power resulting from phase errors. For the case where the VSCF system leads the external AC power source, currents developed by the VSCF system are delivered to the other source, leading to a sagging DC link voltage. If this sag becomes large enough high levels of reactive current can flow and damage the inverter and/or cause tripping of contractors. Another operational difficulty is the inaccurate measurement of output power parameter(s) due to distortion in the output waveform, frequency transients or the like.
Corey, U.S. Pat. No. 3,932,764 discloses a method and apparatus for transferring an electrical load from a utility AC power source to an inverter. The utility AC power source and the inverter are momentarily connected in parallel across the load and the inverter output voltage and phase are controlled to null the current provided by the AC source. Thereafter, the AC source is disconnected from the load to complete the power transfer.